What were the tyres for ride one and two?
Also, isn’t warmer/humid air significantly more dense? I think that’s one of the reasons for racing track in controlled velodromes/at particular altitudes (among other reasons of course).
Warmer air is less dense. See hot air balloons. 
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I just use the free version and never messed with its default settings, Rolling Resistance 0.0050, Drive Train Loss 3%
You can upload a course (TT courses events are already in there) and look at its prediction for a future time/date under various parameters. i.e, for the weekends TT what speed will I go if I up the power, or what speed will I go if I go more aero and lower my CdA, or what balance of both gets the best results etc. I don’t think I’d worry about Rolling Resistance and DT losses not being 100% accurate as they will be constant unlike power/CdA/Weight etc.
I think with the premium version you get more after ride analysis too and a post ride CdA estimate.
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went from cadex classics 25mm to schwable one 28mm. both tubeless.
it definitely doesn’t take 350W to do 23mph.
244W for 20.5 seems a bit high. Anything else change?
- helmet?
- jersey?
- number of bottles?
- amount of water in bottles?
- you said tires, what about wheels?
- I don’t think higher humidity is an advantage
Also, you can get unlucky with the wind. It can increase on your headwind section. It can change directions. The tailwind direction can be blocked by trees/hedges and the headwind section can be wide open.
I’m guessing the route isn’t perfectly flat. Because of the exponential nature of wind drag, there’s probably a smart and less smart way to pace yourself. eg, doing high watts on the uphill section vs high watts on the downhill section. One might result in a higher ave speed than the other.
Do you have your computer set to auto-pause? It takes a LOT of power to get back up to speed from zero so you already face a disadvantage each time you stop. Having those stops also whittle down your ave speed is a double whammy. Yes, you rest at a redlight but you still lose more speed than if you caught a green light.
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Nothing really changed. Same helmet and jersey. was carrying less water given the short duration of the race on the second run. Same wheels. Higher humidity/temp has definitely been an advantage for me in the past, but can’t confirm scientifically. Since I added mywindsock data, the weather conditions/wind were more favorable on run 2.
Still unsure why the speed wasn’t faster. The only change with an uncertain impact was the tires. I just find it hard to believe they could seemingly be so much slower. On that point I can’t find rolling resistance data online for Cadex classics or Schwable ones (not the pros).
Those tires have significantly different (percentage wise) volume (25 mm vs 28 mm). Did you run them at the same pressure? You shouldn’t. Bigger tires should be run at lower pressure, otherwise it’s actually slower.
Fewer bottles can be slower due to some frames being more aero with the bottles vs without. Some people run an empty bottle.
Also, how are you determining average speed? Is that from GPS, wheel sensor or dividing course distance by time? A 10 mile course isn’t really 10 miles for everyone. Every deviation side to side from the shortest path adds distance. This isn’t captured well by GPS or course distance divided by time.
Anyways, overall, the most likely thing is you were less aero the second time. It’s WAY more complex then in the drops or not or what equipment you wore. Very slight changes in body position matter a lot. Aero is so hard to predict and quantify that you’ll never be able to prove if that was it or not. But if it’s not the other stuff, that’s most likely it.
I ran the tires at lower pressure and speed was measured with a wheel speed sensor, so should be reasonably accurate and consistent ride to ride (but I am sure not perfect!).